Photoacoustic detector unit, photoacoustic sensor and associated production methods

ABSTRACT

A photoacoustic detector unit comprises a housing having an opening, and also a photoacoustic transducer designed to convert optical radiation into at least one from a pressure signal or a heat signal. The photoacoustic transducer covers the opening of the housing, such that the photoacoustic transducer and the housing form an acoustically tight cavity. A pressure pick-up is arranged in the acoustically tight cavity.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to German Patent Application No.102019134267.8 filed on Dec. 13, 2019, the content of which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to photoacoustic detector units,photoacoustic sensors and associated production methods.

BACKGROUND

Photoacoustic sensors can detect specific gas species in the ambientair, for example. In particular, harmful or hazardous components in theambient air can be detected in this case. The correct functioning ofsuch sensors can thus be of extremely high importance in manyapplications, particularly if the sensors are used for the safety ofwork personnel. Photoacoustic sensors can be constructed from aplurality of components and generally consist of an emitter unit and adetector unit.

BRIEF SUMMARY

Implementations described herein may provide photoacoustic detectorunits configured to effectively detect different gas species in theambient air. Furthermore, implementations described herein may providecost-effective methods for producing such photoacoustic detector units.A first aspect relates to a photoacoustic detector unit. Thephotoacoustic detector unit comprises a housing having an opening. Thephotoacoustic detector unit furthermore comprises a photoacoustictransducer designed to convert optical radiation into at least one froma pressure signal or a heat signal, wherein the photoacoustic transducercovers the opening of the housing, such that the photoacoustictransducer and the housing form an acoustically tight cavity. Thephotoacoustic detector unit furthermore comprises a pressure pick-uparranged in the acoustically tight cavity.

A second aspect relates to a photoacoustic sensor. The photoacousticsensor comprises an optical emitter and a photoacoustic detector unit inaccordance with the first aspect.

A third aspect relates to a method. The method comprises bonding a firstwafer composed of a first material to a second wafer composed of asecond material in a reference gas atmosphere, wherein a plurality ofhermetically sealed cavities are formed, which enclose the reference gasof the reference gas atmosphere. The method furthermore comprisessingulating the bonded wafers into a plurality of photoacoustictransducers for a photoacoustic detector unit, wherein each of thephotoacoustic transducers comprises one of the hermetically sealedcavities.

BRIEF DESCRIPTION OF THE DRAWINGS

Photoacoustic detector units, photoacoustic sensors and associatedproduction methods in accordance with the disclosure are explained ingreater detail below with reference to drawings. The elements shown inthe drawings are not necessarily rendered in a manner true to scalerelative to one another. Identical reference signs may designateidentical components.

FIG. 1 shows a schematic view of a photoacoustic sensor 100 inaccordance with the disclosure.

FIG. 2 shows a schematic view of a photoacoustic sensor 200 inaccordance with the disclosure.

FIG. 3 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 300 in accordance with the disclosure.

FIG. 4 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 400 in accordance with the disclosure.

FIG. 5 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 500 in accordance with the disclosure.

FIG. 6 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 600 in accordance with the disclosure.

FIG. 7 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 700 in accordance with the disclosure.

FIG. 8 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 800 in accordance with the disclosure.

FIG. 9 schematically illustrates a cross-sectional side view of aphotoacoustic detector unit 900 in accordance with the disclosure.

FIG. 10 schematically illustrates a cross-sectional side view of aphotoacoustic sensor 1000 in accordance with the disclosure.

FIG. 11 illustrates a flow diagram of a method in accordance with thedisclosure.

FIGS. 12A to 12E schematically illustrate a cross-sectional side view ofa method for producing a photoacoustic transducer 1200 for aphotoacoustic detector unit in accordance with the disclosure.

DETAILED DESCRIPTION

The figures described below show photoacoustic detector units,photoacoustic sensors and associated production methods in accordancewith the disclosure. In this case, the described devices and methods maybe illustrated in a general way in order to describe aspects of thedisclosure qualitatively. The devices and methods described may havefurther aspects that may not be illustrated in the respective figure forthe sake of simplicity. However, the respective example can be extendedby aspects described in association with other examples in accordancewith the disclosure. Consequently, explanations concerning a specificfigure may equally apply to examples of other figures.

The photoacoustic sensor or photoacoustic gas sensor 100 in FIG. 1 cancomprise a photoacoustic emitter unit 2 and a photoacoustic detectorunit 4. The photoacoustic detector unit 4 can comprise a photoacoustictransducer 6 and a housing 8. The photoacoustic transducer 6 can be acell having a hermetically sealed cavity 10, in which a reference gas 12can be enclosed. The photoacoustic transducer 6 can have an opticallytransparent window 18 on a first side and a membrane 20 on a second sidesituated opposite the first side. The photoacoustic transducer 6 cancover an opening of the housing 8, such that the photoacoustictransducer 6 and the housing 8 can form an acoustically tight cavity 14.A pressure pick-up 16 can be arranged in the acoustically tight cavity14.

Furthermore, a protective gas can optionally be enclosed in theacoustically tight cavity 14. The protective gases specified in thisdescription can be, for example, nitrogen or a noble gas, such as e.g.argon, xenon, krypton. Furthermore, depending on the application, one ormore alternative or additional components can be arranged in theacoustically tight cavity 14, for example one or more from a pressurepick-up ASIC, a photodetector, a photodiode, a temperature sensor, anoptical emitter.

The photoacoustic emitter unit 2 can be a broadband emitter, which canbe designed to emit optical radiation over a wide frequency range. Inother words, the radiation emitted by the broadband emitter can comprisenot just predetermined frequencies or predetermined frequency bands. Theterm “optical radiation” used in this description can generally refer toa partial range of the electromagnetic spectrum having wavelengths ofbetween approximately 100 nm and approximately 100 μm. That is to saythat the optical radiation can comprise, in particular, at least onefrom the following: ultraviolet radiation having a wavelength ofapproximately 100 nm to approximately 380 nm, infrared radiation havinga wavelength of approximately 780 nm to approximately 100 μm, orradiation having a wavelength of approximately 780 nm to approximately 5μm, e.g. near-infrared radiation and portions of mid-infrared radiation.The last-mentioned range can comprise, inter alia, the absorptionlines/bands of carbon dioxide at 4.26 μm and of further gas species.Even more specifically, the optical radiation can have a wavelength ofapproximately 300 nm to approximately 20 μm.

The photoacoustic emitter unit 2 can be designed to emit optical pulseshaving a predetermined repetition frequency and one or morepredetermined wavelengths. In this case, a predetermined wavelength cancomprise an absorption band of a gas to be detected or of the referencegas 12. The repetition frequency of the optical pulses can be within alow-frequency range or within a frequency range of approximately 1 Hz toapproximately 10 kHz, in particular of approximately 1 Hz toapproximately 1 kHz. Even more specifically, a typical frequency rangecan be between approximately 1 Hz and approximately 100 Hz,corresponding to a pulse duration range of approximately 0.01 s toapproximately 1 s.

A manner of functioning of the photoacoustic sensor 100 is describedbelow. The optical pulses emitted by the emitter unit 2 can pass throughan interspace 22 situated between the emitter unit 2 and the detectorunit 4. By way of example, the interspace 22 can be filled with ambientair. During propagation through the interspace 22, the optical pulsescan be at least partly absorbed by portions of a gas to be detected ifsuch a gas is present in the interspace 22 (e.g. in the ambient air).The absorption can be specific to the gas to be detected, e.g.characteristic rotation or vibration modes of atoms or molecules of thegas to be detected.

The optical pulses can pass through the material of the opticallytransparent window 18 and impinge on atoms or molecules of the referencegas 12 in the hermetically sealed cavity 10. The reference gas 12 cancorrespond to the gas to be detected. The reference gases mentioned inthis description can be, for example, carbon dioxide, nitrogen oxide,methane, ammonia. The optical pulses can at least partly be absorbed bythe reference gas 12 and bring about local pressure increases in thereference gas 12. The pressure increases can be passed on to themembrane 20 and through the latter into the acoustically tight cavity14. In other words, the photoacoustic transducer 6 can be designed toconvert optical radiation in the form of e.g. optical pulses intopressure signals. The photoacoustic transducer 6 is acoustically coupledto the acoustically tight cavity 14.

As an alternative or in addition to the pressure signals described, thephotoacoustic transducer 6 can convert the optical radiation into heatsignals. In this context, the photoacoustic transducer 6 can also bereferred to as a photothermal transducer. In this case, the membrane 20can be heated by absorption of the optical pulses, in particular withthe predetermined repetition frequency of the optical pulses. As aresult of the periodic heating and cooling of the membrane 20, pressurechanges can be produced in the downstream acoustically tight cavity 14,which pressure changes can be detected by the pressure pick-up 16.

Generally, the photoacoustic transducers in accordance with thedisclosure as described herein can accordingly convert optical radiationinto at least one from a pressure signal or a heat signal. In this case,the type of signal generated can be dependent on the respectiveconfiguration of the photoacoustic transducer. A conversion into apressure signal can be provided in particular by way of an deflection ormechanical bending of the membrane, while a conversion into a heatsignal can be provided in particular by heating and cooling of themembrane. Depending on the configuration of the respective photoacoustictransducer, a conversion into a pressure signal and/or a heat signal cantake place. Pressure signals and heat signals generated can both bedetected by a downstream acoustically tight cavity with pressurepick-up. Furthermore, the pressure signals can also be detected in themembrane itself, for example by one or more piezo-elements integratedinto the membrane.

The expression “acoustically tight” used for the cavity 14, for example,need not necessarily mean in this description that the cavity 14 ishermetically or completely sealed. Rather, the walls forming the cavity14 can be designed to provide, during operation of the photoacousticsensor 100, pressure equalization with the surroundings such that thepressure pick-up 16 can be operated properly. In this case, it cannot beexcluded, for example, that the walls of the cavity 14 have one or moresmall openings which do not influence, or which influence onlynegligibly, the pressure equalization for proper operation. The term“acoustically tight” can optionally be replaced by the term“semi-hermetic”.

The pressure signals and/or heat signals passed on by the membrane 20can be detected by the pressure pick-up 16 in the acoustically tightcavity 14. The pressure pick-ups specified in this description can be,for example, microphones or any other type of pressure sensors orpressure-sensitive sensors. The signals detected by the pressure pick-up16 can be processed logically by one or more circuits. By way ofexample, such signal processing can be carried out by an ASIC.

If no portions of a gas to be detected are present in the interspace 22or in the ambient air, the optical pulses emitted by the emitter unit 2are merely absorbed by the reference gas 12 and the pressure pick-up 16will detect a periodic measurement signal with the repetition frequencyof the optical pulses and a first amplitude. If, in contrast thereto,portions of a gas to be detected are present in the interspace 22, theoptical radiation can additionally be absorbed by the portions. Thepressure pick-up 16 will then output a periodic measurement signalhaving a second amplitude, which can be smaller than the firstamplitude. A presence and/or a concentration of the gas to be detectedin the ambient air can be determined on the basis of the magnitudes andprofiles of the first and second amplitudes. If the concentration of thegas to be detected exceeds a predetermined threshold value, for examplea signal, in particular a warning signal, can be output by thephotoacoustic sensor 100 or a device connected thereto.

Using a broadband emitter 2 and a photoacoustic transducer 6 containingthe species of a gas to be detected in its cavity 10, any gas specieswhose absorption bands lie in the spectrum of a black body radiator canbe detected by the photoacoustic sensor 100 in FIG. 1 .

In conventional photoacoustic sensors, the pressure pick-up and thereference gas can be arranged in a common hermetically sealed cavity.Sealing the cavity and simultaneously filling the cavity with thereference gas can be demanding in terms of process engineering. Incontrast thereto, the reference gas 12 in accordance with the presentdisclosure can be arranged in the cell of the photoacoustic transducer6. As a result, during the production of the photoacoustic sensor 100,the process steps mentioned can be decoupled from mounting the pressurepick-up 16 in the cavity 14.

In the case of the conventional photoacoustic sensors, the photoacousticconversion can be provided in particular in the common cavity in whichthe reference gas and the pressure pick-up are arranged. In contrastthereto, in the case of the photoacoustic sensor 100 in accordance withthe disclosure, the photoacoustic conversion can be provided in aseparate hermetically sealed cavity 10 disposed upstream of theacoustically tight cavity 14 with the pressure pick-up 16 arrangedtherein. In accordance with the disclosure, the cavities 10 and 14 withreference gas 12 and pressure pick-up 16, respectively, can be decoupledfrom one another.

In the case of the described construction of the photoacoustic sensor100, the pressure pick-up 16 can have an extremely high sensitivity, asa result of which an extremely high sensitivity of the photoacousticsensor 100 can be provided. As a result, it is possible to achieve areduced energy consumption during operation of the photoacoustic sensor100.

It is evident from the method in FIGS. 12A-E described further belowthat the photoacoustic sensor 100 or the photoacoustic transducer 6 canbe produced on the basis of cost-effective method steps at the waferlevel.

The photoacoustic sensor or photoacoustic gas sensor 200 in FIG. 2 cancomprise a photoacoustic emitter unit 2 and a photoacoustic detectorunit 4. The units 2 and 4 can be spaced apart from one another by one ormore spacers 24, as a result of which an interspace 22 arranged betweenthe units 2 and 4 can be formed. An optical filter 40 can be arrangedbetween the photoacoustic emitter unit 2 and the photoacoustic detectorunit 4. The photoacoustic emitter unit 2 can comprise a housing 26having an cavity 28, in which an emitter 30 and a protective gas 32A canbe arranged. The photoacoustic detector unit 4 can comprise a housing 34having an cavity 36. A photoacoustic transducer 6 in the form of amembrane 70, a pressure pick-up 16 and a pressure pick-up ASIC 38 can bearranged in the acoustically tight cavity 36. The components of thephotoacoustic sensor 200 can be similar to corresponding components ofthe photoacoustic sensor 100 in FIG. 1 , such that explanationsconcerning FIG. 1 can also apply to FIG. 2 .

A manner of functioning of the photoacoustic sensor 200 is describedbelow. The emitter 30 can emit optical radiation, in particular in theform of optical pulses. In this case, the emitter 30 can be for examplea broadband emitter that emits optical radiation over a wide frequencyrange. The (broadband) radiation emitted by the emitter 30 can firstlypass through the protective gas 32A and the housing 26. In this case,the housing 26 can be fabricated from a material that is transparent tothe optical radiation, for example from IR-transparent silicon. Theemitted radiation can be filtered by the optical filter 40 and passthrough the interspace 22. In this case, the optical filter 40 can be orcomprise an optical bandpass filter, in particular. The optical bandpassfilter 40 can be transmissive to optical radiation having a wavelengthwhich can comprise an absorption band of a gas to be detected. Uponpassing through the interspace 22 or the ambient air, the filteredoptical radiation can impinge on portions of a gas to be detected if theambient air contains such portions.

The optical radiation can pass through the upper part of the housing 34and enter the cavity 36. In this case, at least the upper part of thehousing 34 can be fabricated from a material that is transparent to theoptical radiation, for example from IR-transparent silicon. In thecavity 36, the optical radiation can impinge on the membrane 70, whichcan have a low thermal mass, in particular. The membrane 70 can absorbthe optical radiation and thereby produce pressure changes in the cavity36 lying below the membrane 70. The pressure changes can be detected bythe pressure pick-up 16. The signals detected by the pressure pick-up 16can be processed logically by the pressure pick-up ASIC 38.

As already described in association with FIG. 1 , the signals detectedby the pressure pick-up 16 can depend on whether or not portions of thegas to be detected are present in the interspace 22 or the ambient air.A presence and/or a concentration of the gas to be detected in theambient air can be determined on the basis of the signals detected.

The photoacoustic sensor 200 can be operated without the use of areference gas. With the use of a broadband emitter 30 and a suitableoptical filter 40, it is possible to detect any gas species in thespectrum of a black body radiator using the photoacoustic sensor 200. Inthis case, the gas selectivity need not necessarily be provided by thechoice of a reference gas, but rather can be provided by the opticalfilter property of the photoacoustic emitter unit 2 and/or of theoptical filter 40.

In the case of the described construction of the photoacoustic sensor200, the pressure pick-up 16 can have an extremely high sensitivity, asa result of which an extremely high sensitivity of the photoacousticsensor 200 can be provided. As a result, it is possible to achieve areduced energy consumption during operation of the photoacoustic sensor200.

The photoacoustic sensor 200 can be produced on the basis ofcost-effective method steps at the wafer level.

The photoacoustic detector unit 300 in FIG. 3 can for example be used inthe photoacoustic sensor 100 in FIG. 1 and comprise similar components.With regard to operation of the photoacoustic detector unit 300,reference is made to corresponding explanations concerning FIG. 1 .

The photoacoustic detector unit 300 can comprise a photoacoustictransducer 6, which can comprise an optically transparent window 18 anda membrane 20. The optically transparent window 18 and the membrane 20can form a hermetically sealed cavity 10, which can enclose a referencegas 12. In one example, the optically transparent window 18 can befabricated from IR-transparent silicon. The membrane 20 can befabricated from a glass material, for example from a borosilicate. Themembrane 20 can be designed to absorb optical radiation such as e.g. IRradiation. As a result of the absorption, the membrane 20 can be heatedand generate a heat signal. In other words, the optical radiation can beconverted into a heat signal by the membrane 20. On account of theperiodic heating and cooling of the membrane 20, pressure changes can beproduced in an acoustically tight cavity 14 arranged below the membrane20. The pressure changes can be detected by a pressure pick-up 16.

The optically transparent window 18 and the membrane 20 can be securedto one another by way of an anodic bond connection 42. It is evidentfrom the method in FIGS. 12A-E as described further below that anodicbonding of the optically transparent window 18 and the membrane 20 canbe carried out at the wafer level. An antireflection coating 44 can bearranged on the top side of the window 18, and can be designed tosuppress reflection of optical radiation that can be provided by aphotoacoustic emitter unit (not illustrated). The transmission of theoptically transparent window 18 can be increased by the antireflectioncoating 44.

The photoacoustic detector unit 300 can furthermore comprise a housing8, which can form the shape of a shell or a trough. In one example, thehousing 8 can be fabricated from a mold compound. The mold compound caninclude at least one from an epoxy, a filled epoxy, a glass-fiber-filledepoxy, an imide, a thermoplastic, a thermosetting polymer, a polymermixture. The photoacoustic transducer 6 can cover an opening on the topside of the housing 8, wherein the housing 8 and the photoacoustictransducer 6 can form the acoustically tight cavity 14. In FIG. 3 , thephotoacoustic transducer 6 and the housing 8 can be connected to oneanother by an adhesive 46, for example. A protective gas can optionallybe enclosed in the acoustically tight cavity 14.

The pressure pick-up 16 can be arranged on the bottom surface of thehousing 8. The pressure pick-up can be a microphone chip, for example,which can comprise one or more MEMS structures and/or movablestructures. Furthermore, the microphone chip of pressure pick-up 16 caninclude an ASIC for logically processing the signals detected by theMEMS structures. The microphone chip of pressure pick-up 16 can beelectrically connected to one or more connecting conductors 50 by way ofone or more electrical connection elements 48. In the example in FIG. 3, the electrical connection element 48 is illustrated as a bond wire,for example. The connecting conductors 50 can extend through the housing8 and provide an electrical connection between the microphone chip ofpressure pick-up 16 and further components (not illustrated) arrangedoutside the housing 8.

The photoacoustic detector unit 400 in FIG. 4 can be used for example inthe photoacoustic sensor 100 in FIG. 1 . Furthermore, the photoacousticdetector unit 400 can at least partly be similar to the photoacousticdetector unit 300 in FIG. 3 and comprise identical components.

In contrast to FIG. 3 , the pressure pick-up 16 in FIG. 4 can beembodied or arranged in a different way. In this case, the pressurepick-up 16 or its MEMS structures can be arranged in particular suchthat they lie outside a course of the optical radiation provided by aphotoacoustic emitter unit (not illustrated). Signals detected by thepressure pick-up 16 can be corrupted by optical radiation impinging onthe MEMS structures of the pressure pick-up 16. On account of thearrangement of the pressure pick-up 16 outside the optical path as shownin FIG. 4 , such corruption can be avoided or at least reduced.

The photoacoustic detector unit 400 can comprise a pressure pick-updevice 52. The pressure pick-up device 52 can comprise a circuit boardor a substrate 54 with a pressure pick-up 16 and pressure pick-up ASIC38 arranged on the underside of the circuit board or the substrate 54.The pressure pick-up 16 and the pressure pick-up ASIC 38 can beelectrically connected to one another by way of one or more bond wires56, for example. Furthermore, the pressure pick-up 16 and the pressurepick-up ASIC 38 can be electrically coupled to the connecting conductors50 by way of one or more bond wires 58, by way of a wiring layer 60within the circuit board or the substrate 54 and by way of theelectrical connecting elements 48. The pressure pick-up device 52 cancomprise a cover 62 having an opening 64, the cover being arranged overthe pressure pick-up 16 and over the pressure pick-up ASIC 38.

The photoacoustic detector unit 500 in FIG. 5 can be used for example inthe photoacoustic sensor 100 in FIG. 1 . Furthermore, the photoacousticdetector unit 500 can at least partly be similar to the photoacousticdetector unit 300 in FIG. 3 and comprise identical components.

In contrast to FIG. 3 , the photoacoustic detector unit 500 can compriseone or more metal layers and/or metal alloy layers 66, which can bearranged on the membrane 20. In the example in FIG. 5 , a metal layer 66can be arranged in each case on the top side and on the underside of themembrane 20. In further examples, a metal layer can be arranged only onthe top side or only on the underside of the membrane 20. In the examplein FIG. 5 , only one metal layer 66 in each case is arranged on the topside and underside. In further examples, a layer stack having aplurality of metal layers stacked one above another can be arranged onthe respective side of the membrane 20. In the example in FIG. 5 , therespective metal layer 66 can cover substantially the entire exposedsurface of the membrane 20. In further examples, the respective metallayer 66 can cover only selected parts of the membrane surfaces. A metallayer 66 arranged on the membrane 20 can have a lower heat capacity incomparison with the membrane 20.

The photoacoustic detector unit 600 in FIG. 6 can for example be used inthe photoacoustic sensor 200 in FIG. 2 and comprise similar components.With regard to operation of the photoacoustic detector unit 200,reference is made to corresponding explanations concerning FIG. 2 .

The photoacoustic detector unit 600 can comprise a housing 34 with apressure pick-up 16 arranged therein. A photoacoustic transducer 6 inthe form of a membrane 70 can cover an upper opening of the housing 34and form with the latter an acoustically tight cavity 36. The membrane70 can have an elastic inner region 72 and a thicker edge region 74. Theedge region 74 can have the shape of a frame. The inner region 72 can besuspended from or secured to the edge region 74 and be designed tooscillate in the y-direction. As viewed in the y-direction, the innerregion 72 can have a circular shape, for example. In the example in FIG.6 , the membrane 70 can be fabricated from a glass material, for examplefrom a borosilicate. The inner region 72 of the membrane 70 can have atits outer regions one or more ventilation holes 68, which can resultfrom a structured suspension of the inner region 72 from the edge region74 of the membrane 70. In the example in FIG. 6 , a metal layer 66 canbe arranged on the underside of the membrane 70. In a further example, afurther metal layer can be arranged on the top side of the membrane 70.

The photoacoustic detector unit 700 in FIG. 7 can be used for example inthe photoacoustic sensor 200 in FIG. 2 . Furthermore, the photoacousticdetector unit 700 can for example at least partly be similar to thephotoacoustic detector unit 600 in FIG. 6 and comprise identicalcomponents.

The photoacoustic detector unit 700 can comprise a photoacoustictransducer 6 in the form of a membrane 70. In contrast to FIG. 6 , themembrane 70 can be fabricated from a doped semiconductor material. Onaccount of the doping of the semiconductor material, the membrane 70 canbe designed to absorb optical radiation and to convert it into at leastone from a pressure signal or a heat signal. In one example, themembrane 70 can be fabricated from silicon and be doped with at leastone from boron, phosphorus, aluminum, indium, arsenic, antimony.

In a further contrast to FIG. 6 , the photoacoustic transducer 6 canfurthermore comprise an optically transparent cover 76, which can beconnected to the membrane 70 and can form with the latter an (inparticular hermetically sealed) cavity 10. The cover 76 can befabricated from silicon, for example. If the cover 76 and the membrane70 are fabricated from silicon, they can be secured to one another byway of a eutectic silicon-silicon bond connection, for example. Asevident from the method in FIGS. 12A-E, it is possible to carry outeutectic bonding using an intermediate layer at the wafer level. Theintermediate layer can be fabricated from gold, for example.

In yet another contrast to FIG. 6 , the photoacoustic transducer 6 canoptionally comprise an optical filter layer 78, which can be arrangedfor example on the top side of the cover 76. The optical filter layer 78can be transmissive to electromagnetic radiation in a predeterminedwavelength range. The wavelength range can comprise, in particular, anabsorption band of a gas to be detected.

A reference gas can optionally be enclosed in the hermetically sealedcavity 10. In this case, the photoacoustic detector unit 700 in FIG. 7can be used for example in the photoacoustic sensor 100 in FIG. 1 .

The photoacoustic detector unit 800 in FIG. 8 can at least partly besimilar to the photoacoustic detector unit 700 in FIG. 7 and compriseidentical components, such that explanations concerning FIG. 7 can alsoapply to the photoacoustic detector unit 800.

In contrast to FIG. 7 , the photoacoustic detector unit 800 canadditionally comprise one or more piezo-elements 80 integrated into themembrane 70. In this case, the piezo-elements 80 can be arranged forexample at the edge region of the membrane 70 or at a suspension of theinner region of the membrane 70. The piezo-elements 80 can be designedto provide an electrical signal designed as a reference signal for ameasurement signal provided by the pressure pick-up 16. By way ofexample, undesired acoustic influences that can occur during operationof the photoacoustic detector unit 800 can be averaged out on the basisof a comparison of the measurement signal with the reference signal. Themembrane 70 can have electrical contact pads 82 on its underside, by wayof which reference signals generated by the piezo-elements 80 can beprovided.

In a further contrast to FIG. 7 , the housing 34 can be embodied in adifferent way. The housing 34 in FIG. 8 can be produced from a ceramicmaterial, for example. In the cross-sectional side view in FIG. 8 , thehousing 34 can have a stepped shape. The pressure pick-up 16 can bearranged on the bottom surface of the housing 34. Electrical contactpads 84 can be arranged on the top sides of the steps, and can beelectrically connected to via connections 86 extending perpendicularlythrough the housing 34. Further contact pads 88 can be arranged on theunderside of the housing 34. Reference signals detected by thepiezo-elements 80 can be forwarded to one or more of the contact pads 88by way of the contact pads 82 and by way of the via connections 86. In asimilar way, measurement signals of the pressure pick-up 16 can beforwarded to one or more of the contact pads 88 by way of the contactpads 84 and the via connections 86.

The photoacoustic detector unit 900 in FIG. 9 can be at least partlysimilar to the photoacoustic detector unit 700 in FIG. 7 . In contrastto FIG. 7 , the photoacoustic transducer 6 can comprise an additionalintermediate layer 90, which can be arranged between the membrane 70 andthe cover 76. The intermediate layer 90 can be designed to simplifyconnection of the membrane 70 to the cover 76 in terms of processengineering. By way of example, the membrane 70 can be fabricated fromdoped silicon and the cover 76 can be fabricated from silicon. In such acase, the intermediate layer 90 can be fabricated from a glass material,in particular a borosilicate. As a result, the membrane 70 and the cover76 can be connected to the intermediate layer 90 in each case by anodicbonding. Furthermore, a use of the intermediate layer 90 makes itpossible to adapt or increase the structural height of the photoacoustictransducer 6 in the y-direction in a simple manner.

The photoacoustic sensor 1000 in FIG. 10 can be similar to one of thephotoacoustic sensors 100 and 200 in FIGS. 1 and 2 . In particular, theconstruction shown in FIG. 10 can be used for a realization of thephotoacoustic sensors in FIGS. 1 and 2 .

The photoacoustic sensor 1000 in FIG. 10 can comprise a photoacousticemitter unit 2 and a photoacoustic detector unit 4. A spatial separationof the units 2 and 4 is indicated qualitatively in FIG. 10 by aperpendicularly extending dashed line. In the example in FIG. 10 , thephotoacoustic detector unit 4 can for example be similar to thephotoacoustic detector unit 700 in FIG. 7 , such that in this regardreference can be made to explanations concerning FIG. 7 .

The photoacoustic sensor 1000 can comprise a housing 8, which can beseparated into a left and right part by a separating structure 92. Inthis case, the right part of the housing 8 can correspond to the housing34 in FIG. 7 . An optical emitter 30 can be arranged on the left part ofthe housing 8. Depending on the implementation of the photoacousticdetector unit 4, the optical emitter 30 can be a broadband emitter withor without a downstream optical bandpass filter. The photoacousticsensor 1000 can furthermore comprise a cover 94 having an opticallyreflective inner surface, the cover being arranged above the units 2 and4.

During operation of the photoacoustic sensor 1000, the emitter 30 canemit optical radiation that can propagate along an optical pathrepresented by three arrows in FIG. 10 . The emitter 30 can emit opticalradiation in the direction of the cover 94. The emitted radiation can bereflected at the inner surface of the cover 94. In order to be able toprovide the reflection course illustrated qualitatively in FIG. 10 , theinner surface of the cover 94 can be shaped in a suitable manner. Theoptical radiation reflected from the inner surface of the cover 94 canimpinge on the photoacoustic detector unit 4.

FIGS. 12A-E illustrate a flow diagram of a method in accordance with thedisclosure. By way of example, one or more photoacoustic transducers fora photoacoustic detector unit in accordance with the disclosure can beproduced with the aid of the method.

At 96 a first wafer composed of a first material is bonded to a secondwafer composed of a second material in a reference gas atmosphere. Inthis case, a plurality of hermetically sealed cavities are formed, whichenclose the reference gas of the reference gas atmosphere. At 98 thebonded wafer is singulated into a plurality of photoacoustic transducersfor a photoacoustic detector unit. In this case, each of thephotoacoustic transducers comprises one of the hermetically sealedcavities.

The method in FIGS. 12A-E can be regarded as a more detailedimplementation of the method in FIG. 11 . In FIG. 12A, a first wafer 102composed of a first material can be provided. The first wafer 102 canhave a multiplicity of depressions 104. In this case, the number ofdepressions 104 can correspond, in particular, to a number ofphotoacoustic transducers to be produced by the method in FIGS. 12A-E.In the cross-sectional side view in FIG. 12A, the depressions 104 canhave a rounded shape. In further examples, the shape of the depressions104 can be chosen differently, for example square, rectangular,polygonal, etc. The first wafer 102 can be fabricated from a glassmaterial or a doped semiconductor material, for example.

In FIG. 12B, a second wafer 106 composed of a second material can beprovided. The second wafer 106 can have a multiplicity of depressions108. The number of depressions 108 can correspond in particular to thenumber of depressions 104 of the first wafer 102. In the example in FIG.12B, the depressions 108 can have a rounded shape. In further examples,the shape of the depressions 108 can be chosen differently, for examplesquare, rectangular, polygonal, etc. The second wafer 106 can befabricated from a semiconductor material, for example.

In FIG. 12C, the first wafer 102 can be bonded to the second wafer 106in a reference gas atmosphere. For the bonding process, the first wafer102 and the second wafer 106 can be arranged in a bonding chamber (notillustrated) designed to provide the reference gas atmosphere. Duringthe bonding of the wafers 102 and 106, a plurality of hermeticallysealed cavities 110 can be formed, which enclose the reference gas ofthe reference gas atmosphere.

The bonding process employed in FIG. 12C can be dependent in particularon the materials of the wafers 102 and 106. In a first example, thematerial of the first wafer 102 can comprise a glass material (e.g. aborosilicate) and the material of the second wafer 106 can comprise asemiconductor material (e.g. silicon). In this case, the bonding processcan comprise anodic bonding. In a second example, the material of thefirst wafer 102 can comprise a doped semiconductor material (e.g. dopedsilicon) and the material of the second wafer 106 can comprise asemiconductor material (e.g. silicon). In this case, the bonding processcan comprise eutectic bonding using an intermediate layer. Theintermediate layer can be fabricated from gold, for example.

In FIG. 12D, the arrangement from FIG. 12C can be singulated into aplurality of arrangements along perpendicular dashed lines. Thesingulating process can include for example an etching process, a plasmadicing process, a mechanical ultrasonic dicing process, a laser dicingprocess, or a combination thereof.

FIG. 12E shows one of the photoacoustic transducers 1200 obtained as aresult of the singulation, which photoacoustic transducer can comprisean optically transparent window 18 and a membrane 20. In this case, themembrane 20 can be fabricated from the material of the first wafer 102and the optically transparent window 18 can be fabricated from thematerial of the second wafer 106.

The method in FIGS. 12A-E can comprise further steps, which are notexplicitly illustrated and discussed for the sake of simplicity. Thefurther steps can be carried out here in particular at the wafer level.By way of example, the method can be extended by a step in which anantireflection coating can be applied on the second wafer 106, such thatthe photoacoustic transducers 1200 produced can each have anantireflection coating on the optically transparent window 18.

EXAMPLES

Photoacoustic detector units, photoacoustic sensors and associatedproduction methods are explained below on the basis of examples.

Example 1 is a photoacoustic detector unit, comprising: a housing havingan opening; a photoacoustic transducer designed to convert opticalradiation into at least one from a pressure signal or a heat signal,wherein the photoacoustic transducer covers the opening of the housing,such that the photoacoustic transducer and the housing form anacoustically tight cavity; and a pressure pick-up arranged in theacoustically tight cavity.

Example 2 is a photoacoustic detector unit according to example 1,wherein the photoacoustic transducer is designed to convert at least onefrom infrared radiation or ultraviolet radiation into at least one froma pressure signal or a heat signal.

Example 3 is a photoacoustic detector unit according to example 1 or 2,wherein the photoacoustic transducer comprises: a cell having ahermetically sealed cavity; and a reference gas enclosed in thehermetically sealed cavity, wherein the reference gas is designed toabsorb the optical radiation.

Example 4 is a photoacoustic detector unit according to example 3,wherein the cell comprises: an optically transparent window on a firstside of the cell; and a membrane on a second side of the cell, thesecond side being situated opposite the first side.

Example 5 is a photoacoustic detector unit according to example 4,wherein the optically transparent window is fabricated from silicon.

Example 6 is a photoacoustic detector unit according to example 4 or 5,wherein the membrane is fabricated from a glass material.

Example 7 is a photoacoustic detector unit according to any of examples4 to 6, wherein the membrane is fabricated from doped silicon.

Example 8 is a photoacoustic detector unit according to any of examples4 to 7, wherein the optically transparent window and the membrane formthe hermetically sealed cavity.

Example 9 is a photoacoustic detector unit according to any of examples4 to 8, wherein the optically transparent window and the membrane arewafer-bonded.

Example 10 is a photoacoustic detector unit according to any of examples4 to 9, furthermore comprising: an antireflection coating arranged onthe optically transparent window.

Example 11 is a photoacoustic detector unit according to any of examples4 to 10, furthermore comprising: a metal layer arranged on the membrane.

Example 12 is a photoacoustic detector unit according to any of thepreceding examples, furthermore comprising: a protective gas enclosed inthe acoustically tight cavity.

Example 13 is a photoacoustic detector unit according to any of thepreceding examples, wherein the housing is fabricated from a moldcompound.

Example 14 is a photoacoustic detector unit according to example 1,wherein the photoacoustic transducer comprises: a membrane designed toabsorb the optical radiation.

Example 15 is a photoacoustic detector unit according to example 14,wherein the membrane is fabricated from at least one from glass materialor doped silicon.

Example 16 is a photoacoustic detector unit according to example 14 or15, furthermore comprising: a piezo-element integrated into the membraneand designed to provide an electrical signal designed as a referencesignal for a measurement signal provided by the pressure pick-up.

Example 17 is a photoacoustic detector unit according to any of examples14 to 16, furthermore comprising: an optical filter layer, which istransmissive to electromagnetic radiation of a predetermined wavelength,wherein the optical filter layer is applied on at least one from themembrane or a cover arranged above the membrane.

Example 18 is a photoacoustic sensor, comprising: an optical emitter;and a photoacoustic detector unit according to any of the precedingexamples.

Example 19 is a photoacoustic sensor according to example 18, whereinthe optical emitter comprises an optical broadband emitter.

Example 20 is a photoacoustic sensor according to example 19,furthermore comprising: an optical bandpass filter disposed downstreamof the optical broadband emitter, the optical bandpass filter beingtransmissive to electromagnetic radiation of a predetermined wavelength.

Example 21 is a method, comprising: bonding a first wafer composed of afirst material to a second wafer composed of a second material in areference gas atmosphere, wherein a plurality of hermetically sealedcavities are formed, which enclose the reference gas of the referencegas atmosphere; and singulating the bonded wafers into a plurality ofphotoacoustic transducers for a photoacoustic detector unit, whereineach of the photoacoustic transducers comprises one of the hermeticallysealed cavities.

Example 22 is a method according to example 21, wherein: the firstmaterial comprises a glass material, the second material comprises asemiconductor material, and the bonding comprises anodic bonding.

Example 23 is a method according to example 21, wherein: the firstmaterial comprises a doped semiconductor material, the second materialcomprises a semiconductor material, and the bonding comprises eutecticbonding using an intermediate layer.

Although specific implementations have been illustrated and describedherein, it is obvious to the person of average skill in the art that amultiplicity of alternative and/or equivalent implementations canreplace the specific implementations shown and described, withoutdeparting from the scope of the present disclosure. This application isintended to cover all adaptations or variations of the specificimplementations discussed herein. Therefore, the intention is for thisdisclosure to be restricted only by the claims and the equivalentsthereof.

The invention claimed is:
 1. A photoacoustic detector unit, comprising:a housing having an opening; a photoacoustic transducer configured toconvert optical radiation into at least one of a pressure signal or aheat signal, wherein the photoacoustic transducer covers the opening ofthe housing such that the photoacoustic transducer and the housing forman acoustically tight cavity, and wherein the photoacoustic transducercomprises: a cell having a hermetically sealed cavity; and a referencegas enclosed in the hermetically sealed cavity, wherein the referencegas is configured to absorb the optical radiation; and a pressurepick-up arranged in the acoustically tight cavity.
 2. The photoacousticdetector unit as claimed in claim 1, wherein the photoacoustictransducer is configured to convert at least one from infrared radiationor ultraviolet radiation into at least one of a pressure signal or aheat signal.
 3. The photoacoustic detector unit as claimed in claim 1,wherein the cell comprises: an optically transparent window on a firstside of the cell; and a membrane on a second side of the cell, thesecond side being situated opposite the first side.
 4. The photoacousticdetector unit as claimed in claim 3, wherein the optically transparentwindow is fabricated from silicon.
 5. The photoacoustic detector unit asclaimed in claim 3, wherein the membrane is fabricated from a glassmaterial.
 6. The photoacoustic detector unit as claimed in claim 3,wherein the membrane is fabricated from doped silicon.
 7. Thephotoacoustic detector unit as claimed in claim 3, wherein the opticallytransparent window and the membrane form the hermetically sealed cavity.8. The photoacoustic detector unit as claimed in claim 3, wherein theoptically transparent window and the membrane are wafer-bonded.
 9. Thephotoacoustic detector unit as claimed in claim 3, further comprising:an antireflection coating arranged on the optically transparent window.10. The photoacoustic detector unit as claimed in claim 3, furthercomprising: a metal layer arranged on the membrane.
 11. Thephotoacoustic detector unit as claimed in claim 1, further comprising: aprotective gas enclosed in the acoustically tight cavity.
 12. Thephotoacoustic detector unit as claimed in claim 1, wherein the housingis fabricated from a mold compound.
 13. The photoacoustic detector unitas claimed in claim 1, wherein the photoacoustic transducer comprises: amembrane configured to absorb the optical radiation.
 14. Thephotoacoustic detector unit as claimed in claim 13, wherein the membraneis fabricated from at least one of glass material or doped silicon. 15.The photoacoustic detector unit as claimed in claim 13, furthercomprising: a piezo-element integrated into the membrane and configuredto provide an electrical signal configured as a reference signal for ameasurement signal provided by the pressure pick-up.
 16. Thephotoacoustic detector unit as claimed in claim 13, further comprising:an optical filter layer, which is transmissive to electromagneticradiation of a predetermined wavelength, wherein the optical filterlayer is applied on at least one of the membrane or a cover arrangedabove the membrane.
 17. A photoacoustic sensor, comprising: an opticalemitter; and a photoacoustic detector comprising: a housing having anopening; a photoacoustic transducer configured to convert opticalradiation into at least one of a pressure signal or a heat signal,wherein the photoacoustic transducer covers the opening of the housingsuch that the photoacoustic transducer and the housing form anacoustically tight cavity, and wherein the photoacoustic transducercomprises: a cell having a hermetically sealed cavity; and a referencegas enclosed in the hermetically sealed cavity, wherein the referencegas is configured to absorb the optical radiation; and a pressurepick-up arranged in the acoustically tight cavity.
 18. The photoacousticsensor as claimed in claim 17, wherein the optical emitter comprises anoptical broadband emitter.
 19. The photoacoustic sensor as claimed inclaim 18, further comprising: an optical bandpass filter disposeddownstream of the optical broadband emitter, the optical bandpass filterbeing transmissive to electromagnetic radiation of a predeterminedwavelength.